The present invention relates to ultrasonic medical imaging and quantitative measurements and, in particular, to an improved apparatus and method for making ultrasonic measurements of tissue strain and stiffness.
Conventional ultrasonic imaging provides a mapping of ultrasonic echo signals onto an image plane where the intensity of the echo, caused principally by differences in material properties between adjacent tissue types, is mapped to brightness of pixels on the image plane. While such images serve to distinguish structure within the body, they provide limited insight into the physical properties of the imaged materials.
Ultrasonic elastography is a new ultrasonic modality that may produce an image revealing stiffness properties of the tissue, for example, strain under an externally applied stress, Poisson's ratio, Young's modulus, and other common strain and strain-related measurements.
In one type of elastography, termed “quasi-static” elastography, two images of the tissue in two different states of compression, for example, no compression and a given positive compression, may be obtained by the ultrasound device. The tissue may be compressed by a probe (including the transducer itself) or by muscular action or movement of adjacent organs. Strain may be deduced from these two images by computing gradients of the relative shift of the tissue in the two images along the compression axis. Quasi-static elastography is analogous to a physician's palpation of tissue in which the physician determines stiffness by pressing the tissue and detecting the amount of tissue yield (strain) under this pressure.
The process of deducing the shift in tissue under compression may start by computing local correlations between the images and then evaluating differences in echo arrival time for correlated structures before and after compression. Differences in echo arrival time are converted to tissue displacement (or strain, which is displacement normalized by length) at different points within the tissue by multiplying the difference in arrival times by the speed of sound through the tissue.
The tissue strain indirectly provides an approximate measure of stiffness. Tissue that exhibits less strain under compression may be assumed to be stiffer while tissue that exhibits more strain under compression is assumed to be less stiff. This approximation, however, carries with it an assumption of a constant stress field, that is, that the force of compression is uniformly dispersed within the tissue. Generally this is not true, but rather the stress in an elastic body falls off as one moves away from the compressor and increases at stress concentrations near stiff inclusions.
Accordingly, it has been proposed to improve elastographic measurements by comparing the deduced strains against modeled stress fields produced by a finite size compressor in a semi-infinite homogeneous medium. See, for example, Ponnekanti H., Ophir, J., and Cespedes I., 1992, “Axial Stress Distributions Between Coaxial Compressor in Elastography: An Analytical Model”, Ultrasound in Med. & Biol., Vol. 18, No. 8, pp. 667-673.
A similar analytical model has been proposed for stress concentration near inclusions. See, Cespedes I., Ophir J., and Ponnekanti H., 1993, “Elastography: Elasticity Imaging Using Ultrasound with Application to Muscle and Breast In Vivo”, Ultrason. Imag., Vol. 15, pp. 73-88.